Over 21 million US adults (~10% of the population) have type 2 diabetes mellitus (T2DM) and another 29 million have prediabetes. Targeting prediabetes to prevent the development of T2DM would have a substantial impact on the health of millions. Novel interventions to accomplish this are urgently needed. For this R21 Exploratory/Development grant, we propose to examine morning light treatment as a novel, safe, non- pharmacological intervention to improve glucose metabolism in people with prediabetes. The basis for this research is the recent evidence from both laboratory and population studies indicating that circadian misalignment is independently associated with increased risk for T2DM and poorer glucose control. We also have pilot data indicating later circadian timing (later dim light melatonin onset, DLMO) is significantly associated with a higher fasting glucose level. Here we propose to translate these findings into a novel behavioral intervention, morning light treatment, to improve glucose metabolism in people with prediabetes. To date, morning light treatment has been shown to reduce body fat, but glucose metabolism remains to be investigated. We will test a novel head worn light device (Re-Timer(r)) that reduces subject burden by allowing people to be ambulatory during light treatment. We include pilot data demonstrating our ability to measure compliance to light treatment. We will enroll 34 subjects (17 male, 17 female, ages 40-65 years), who will: (1) be obese (BMI>30 kg/m2), (2) have prediabetes (HbA1c 5.7% to <6.5%), (3) have significant circadian misalignment (social jet lag ? 2 h), and (4) be free of moderate to severe obstructive sleep apnea (apnea- hypopnea index<15). We will measure baseline sleep at home (wrist actigraphy) for 1 week, followed by a laboratory session to measure circadian timing (DLMO) and conduct a 3-hour (8-sample) oral glucose tolerance test (OGTT) in the morning. Subjects will then be randomized to 4 weeks of a 1 hour daily morning light treatment using either an active or placebo Re-timer(r). After 4 weeks of light treatment, the laboratory session will be repeated to reassess DLMO and OGTT. Specific Aim 1 is to determine the effect of active versus placebo morning light treatment on glucose metabolism in individuals with prediabetes. Specific Aim 2 is to determine the extent to which circadian phase advances are related to improvements in glucose metabolism. In an Exploratory Aim we will examine other possible mediators (body fat, activity, total caloric intake, sleep fragmentation) that may also explain changes in glucose metabolism. Morning light treatment is already used by several patient groups and the light device is already commercially available. Thus a morning light treatment to improve glucose metabolism would readily translate into clinical practice, as a novel, safe, non-pharmacological intervention that could be added to other lifestyle interventions. This pilot study is the important first step towards the development of a larger randomized clinical trial of morning light treatment to improve glucose metabolism.